Voltage generator with current source compensated for an error current operable over a wide voltage range

ABSTRACT

In one embodiment, a regulator circuit for generating a regulated output voltage Vout has an error amplifier using a pair of bipolar transistors at its front end. The error amplifier compares the regulated output voltage to a reference voltage Vref. A precision current source draws a first current through a user-selected set resistance to generate the desired Vref. The regulator circuit controls a power stage to cause Vout to be equal to Vref. The base current into one of the bipolar transistors normally distorts the current through the set resistance. A base current compensation circuit is coupled to the current source to adjust the first current by a value equal to the base current to offset the base current. Therefore, Vref is not affected by the base current. The error amplifier may be in a linear regulator or a switching regulator. The compensation circuit may be used in other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/813,485, filed Apr. 18, 2013, by Robert Dobkin et al., assignedto the present assignee and incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to voltage regulator integrated circuits and, inparticular, to a base current compensation circuit coupled to an erroramplifier in such an IC.

BACKGROUND

The present invention applies to the error amplifier used in either alinear regulator or a switching regulator. The drawbacks of the priorart will be discussed with reference to a linear regulator.

FIG. 1 illustrates one representative prior art linear regulator 10integrated circuit, which is a negative voltage regulator, although theinvention applies equally to positive voltage regulators. The term“linear regulator” is generally synonymous with a “low dropout (LDO)regulator.” The term “low dropout” refers to the small minimum voltagedifferential that can occur between the input voltage terminal and theregulated output voltage terminal while still achieving regulation.

LDO regulators operate by varying the conductivity of a seriestransistor, connected between the input terminal and the outputterminal, to achieve a predetermined output voltage. The output level ofan operational amplifier (op amp), which is a type of differentialamplifier, controls the conductivity of the series transistor. The opamp is sometimes referred to herein as an error amplifier. Typically,the regulator's output voltage is fed back into one input terminal ofthe op amp, and the conductivity of the series transistor is controlledto match the output voltage to a reference voltage applied to the otherinput of the op amp. The user selects the reference voltage.Alternatively, a divided output voltage is fed back and matched to afixed reference voltage, where the user selects resistors for thedivider to achieve the desired output voltage.

In FIG. 1, the user connects an Rset resistor 15 to a Set pin 16 of theIC to set the output voltage Vout provided at the Vout pin 18. A load istypically connected between the Vout pin 18 and ground. The inputvoltage (a negative voltage in this example, usually Vee) is applied tothe Vin pin 19, so Vout will be somewhere between Vin (plus the dropoutvoltage) and ground. A fixed precision current source 20 supplies afixed current through the Rset resistor 15 to generate a referencevoltage Vref at the inverting input of the op amp 22, being used as anerror amplifier. The output voltage Vout is applied to the non-invertinginput of the op amp 22. The terms inverting and non-inverting simplyrefer to the two branches of the differential amplifier in the op amp22, shown in FIG. 2.

Using an internal current source 20 and Rset resistor 15 to set thereference voltage is preferred to dividing the output voltage andmatching the divided voltage (typically about 1.2 volts) to a fixedbandgap reference voltage source, since, by using the current source,the loop gain and bandwidth of the regulator are not affected by theoutput voltage, and Vout is allowed to go to a very low voltage.

The op amp 22 controls the conductivity of the pass transistor 24 sothat Vout matches Vref.

FIG. 2 is a basic example of one type of op amp 22, which represents aconventional op amp. The input signals Vout and Vref (from FIG. 1) areapplied to NPN transistors 28 and 30. A current source 33 supplies afixed current to the tied emitters of the transistors 28/30. Well knowncircuitry 32 provides a single ended output for driving the passtransistor 24 (FIG. 1). The circuitry 32 may include other differentialamplifiers, current minors, current sources, and other well-knowncircuitry. The pertinent feature of the op amp 22 represented is thebipolar transistor input stage, drawing a base current Ib from thecurrent source 20. The present invention applies to any type of op amphaving a bipolar transistor input stage.

FIG. 3 illustrates the op amp 22 along with a conventional base currentcompensation circuit 40 connected to the base of the transistor 30. Nobase current compensation is needed for the transistor 28. Since thecurrent source 20 current (Isource) is a fixed known value and the userselects the precise Rset resistor value (R) to achieve a desired Vrefequal to (Isource×R), any base current (Ib) into the transistor 30 willincrease the voltage drop across the Rset resistor and distort thedesired Vref value. Accordingly, it is known to provide the compensationcircuit 40 to supply the same Ib current that is drawn by the transistor30 to effectively cancel the effect of the base current drawn by thetransistor 30. Many circuit configurations are possible, and FIG. 3shows a representative example. FIG. 3 may also represent the op amp(error amplifier) in a switching regulator.

Assuming the current source 33 is designed to draw a current of 2I, thecurrent source 42 in the compensation circuit 40 is designed to draw acurrent of I through the transistor 43 (since transistors 28 and 30 areassumed to be drawing equal currents I during regulation). Transistor 43is matched to transistor 30. This will cause Ib to flow through thecurrent minor of transistors 44 and 46. The mirrored current Ib is thenadded to the node coupling Vref to the base of the transistor 30 tocancel the Ib drawn by the transistor 30.

One problem with the conventional base current compensation circuit 40of FIG. 3 is that it requires Vref to be at least about 0.3 volt belowground for proper operation. This limits the allowable Vout range. Forsome applications, a regulated output voltage between −0.3 volt andground is needed. Similarly, in positive voltage regulators, the Vrefcannot go within 0.3 volt of ground without adversely affecting theoperation of the base current compensation circuit. Other types of basecurrent compensation circuits require even more headroom to operate.Further, in a floating configuration (typically used for high voltages),where the ground pin is connected to Vout, the conventional base currentcompensation circuit does not have sufficient headroom to operate.

The problem occurs equally for error amplifiers in linear regulators andswitching regulators.

What is needed is a base current compensation technique for a positiveor negative regulator employing an error amplifier, where thecompensation circuit does not limit the range of output voltageregulation due to the level of Vref.

SUMMARY

In one embodiment, an LDO regulator IC uses an on-chip current sourcewhich, along with a user-selected Rset resistor connected between a Setpin and system ground, sets the reference voltage Vref for the op amp(error amplifier). The op amp uses bipolar transistors in its inputstage. The op amp drives a pass transistor connected between the inputvoltage (Vin) pin and the output voltage (Vout) pin.

In the case of a negative voltage regulator, the reference voltage Vrefis tied to the inverting input of the op amp, and the non-invertinginput of the op amp is tied to the Vout pin. The regulator controls thepass transistor to cause Vout to be substantially equal to the referencevoltage.

A base current compensation circuit directly compensates the currentsource that sets Vref rather than supplies Ib directly to the Vref node(as done in the prior art). Since the compensation circuit may becoupled between the rails of the system or to another voltage sourceindependent of Vref, and does not require any minimum or maximum levelof Vref to operate, the operation of the compensation circuit isindependent of the level of Vref and Vout.

In an example of compensating for an NPN transistor drawing base currentfrom the current source in the Vref circuit, the current source iscompensated to reduce the current drawn through the Rset resistor by Ib.If the op amp uses PNP transistors for its inputs, the target PNPtransistor sources base current into the current source in the Vrefcircuit, and the current source is compensated to increase the currentdrawn through the Rset resistor by Ib.

The base current compensation technique applies equally to positive andnegative voltage linear and switching regulators that set a Vref using acurrent source and an Rset resistor.

Various other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional negative voltage LDO regulator.

FIG. 2 illustrates a generic and conventional op amp that may be used inany linear regulator.

FIG. 3 illustrates a conventional base current compensation circuit foran op amp (error amplifier) in a linear or switching regulator.

FIG. 4 illustrates a negative voltage LDO regulator IC in accordancewith one embodiment of the present invention.

FIG. 5 illustrates additional detail of the current source and op amp ofFIG. 4 in accordance with one embodiment of the present invention.

FIG. 6 illustrates detail of a base current compensation circuit thatcan be used in the circuit of FIG. 5, assuming NPN transistors are usedin the op amp's front end, in accordance with one embodiment of thepresent invention.

FIG. 7 illustrates a positive voltage LDO regulator employing thepresent invention.

FIG. 8 illustrates detail of a base current compensation circuit thatcan be used in the regulator of FIG. 7 in accordance with one embodimentof the present invention

FIGS. 9A and 9B together illustrate an error amplifier circuit for anegative voltage regulator similar to FIGS. 5 and 6 but adding anothererror amplifier circuit that takes over when Vee becomes within onediode drop of Vref (preventing the proper operation of the op amp 22 ofFIG. 5).

FIG. 10 illustrates the invention being incorporated in an erroramplifier for a switching regulator.

Elements that are the same or equivalent are labeled with the samenumeral.

DETAILED DESCRIPTION

FIG. 4 illustrates the invention incorporated in a negative voltage LDOregulator 50, where the Vin pin 52 is connected to a negative powersupply voltage Vin (also called Vee), and the Vout pin 54 is settable toprovide virtually any voltage between ground and Vee minus the dropoutvoltage. A load would normally be connected between the Vout pin 54 andground.

The power supply input terminals of the op amp 58 are connected betweenground and the Vin pin 52. In other embodiments, the positive supplyinput terminal for the op amp 58 may be tied to a positive voltage orVout. The negative voltage input terminal of the precision currentsource 60 is tied to the Vin pin 52. In one embodiment, the currentsource 60 generates 50 microamps. The non-inverting input of the op amp58 is tied to the Vout pin 54. An Rset resistor 62 is connected by theuser to the Set pin 64, which sets a Vref for comparison with Vout. Theseries transistor 66, connected between the Vin pin 52 and the Vout pin54, is controlled to keep Vout substantially equal to Vref.

The op amp 58 may be any conventional op amp having a bipolar transistorfront end, such as shown in FIG. 2 or other types.

Generating the reference voltage Vref using an on-chip current source 60and an Rset resistor 62 is preferable over comparing a divided outputvoltage to a fixed bandgap reference, since operating characteristics ofthe feedback loop are not affected by the output voltage, and Vout isallowed to go to a very low voltage.

All components other than the Rset resistor 62 are on a single chip,which may be packaged in a 3 or 4-pin package.

Since it is assumed that the front end of the op amp 58 is that of FIG.2, using NPN transistors, a base current is drawn by the NPN transistor30 (FIG. 2) driven by Vref. Therefore, some of the current drawn throughthe Rset resistor 62 is due to the base current of the transistor 30,which would normally increase the desired Vref (in this case makes itmore negative). A base current compensation circuit 68 is provided tocause the current source 60 to decrease its specified current by Ib,equal to the transistor's 30 base current, to offset the base current Ibdrawn by the transistor 30. In other words, the current source 60 itselfdraws less current through the Rset resistor 62 to compensate for thetransistor's base current. Therefore, the desired Vref is achieved.

Since the compensation circuit 68 can be powered by any voltage sourceand does not require any differential between Vref and another voltageto operate, the performance of the compensation circuit 68 is notaffected by the level of Vref (which sets Vout). Therefore, Vout may beregulated to be close to or at ground, or close to Vin (depending on thetype of regulator), while the compensation circuit 68 operates normally.

FIG. 5 illustrates one type of current source 60 that may be used in aregulator to set the Vref for the error amplifier. The op amp 22 (erroramplifier) may be any type of op amp with an NPN bipolar transistorfront end and may be the op amp 58 in FIG. 4, where the output of thecircuit 32 drives the pass transistor 66 in FIG. 4.

A differential amplifier 72 has applied to its non-inverting terminal avoltage Vs from a voltage source 74 (referenced to the voltage on theVin pin 52). An output of the amplifier 72 controls the conductivity ofa MOSFET 75 connecting the Set pin 64 (and the Rset resistor 62) to aresistor 76 connected to the Vin pin 52. The top terminal of theresistor 76 is coupled to the inverting terminal of the amplifier 72 sothat the MOSFET 75 is controlled to conduct a current to cause thevoltage drop across the resistor 76 to equal Vs.

To compensate for the base current Ib into the transistor 30, the basecurrent compensation circuit 68 generates a current equal to Ib andapplies it to the node of the resistor 76. Therefore, the voltage dropacross the resistor 76, having a value R, will be raised by Ib×R. Thiswill effectively lower the current generated by the current source 60 byIb to offset the transistor base current through the Rset resistor 62.Hence, Vref (and Vout) is not affected by the base current. This sametechnique can be applied to any type of current source to reduce orincrease (depending on the type of op amp used) the current generated bythe current source to compensate for the base current of the front endtransistor.

Note that no base current compensation is needed for the Vout side ofthe op amp because Vout is regulated to equal Vref.

FIG. 6 illustrates one example of the base current compensation circuit68. The voltage source 80 supplies sufficient voltage for operation ofthe circuit. In the example of FIG. 6, the base current compensationcircuit 68 is the same as the compensation circuit 40 in FIG. 3 exceptfor its connection to the op amp and to a power source. Many othercircuit configurations may be used for the compensation circuit 68.

Assuming the op amp current source 33 in FIG. 5 is designed to draw acurrent of 2I, the current source 82 in the compensation circuit 68 isdesigned to draw a current of I through the transistor 84 (sincetransistors 28 and 30 are assumed to be drawing equal currents I duringregulation). Transistor 84 is matched to transistor 30. This will causeIb to flow through the current minor of transistors 86 and 88. Themirrored current Ib is then added to the node coupling the resistor 76in FIG. 5 to the MOSFET 75 to offset the Ib drawn by the transistor 30.

FIG. 7 illustrates the invention being applied to the op amp 90 (erroramplifier) of a positive voltage LDO regulator 92. In this example, Vrefis a positive voltage set by the value of the Rset resistor 94 and theon-chip current source 96. The op amp 90 controls the PNP passtransistor 98 to cause Vout to be substantially equal to Vref.

It is assumed the front end of the op amp 90 includes NPN transistorssinking a base current Ib from the current source 96. This wouldnormally lower the current through the Rset resistor 94 and lower Vref.However, the base current compensation circuit 100 operates to cause thecurrent source 96 to increase its current by Ib to offset the basecurrent so that the desired Vref is achieved. The current source 96 maybe similar to that shown in FIG. 5, except the polarities would bereversed since Vin would be a positive supply voltage. Since the NPNtransistor in the op amp 90 of FIG. 7 draws current away from the Rsetresistor 94, the compensation circuit 100 needs to sink the current Ibaway from the resistor 76 node (FIG. 5) to cause the MOSFET 75 to turnon more in order for the voltage drop across the resistor 76 to equalthe voltage Vs. This increased current from the current source 96 inFIG. 7 thus compensates for the base current.

FIG. 8 illustrates one embodiment of the compensation circuit 100 thatsinks the current Ib from the resistor 76 (FIG. 5) node to cause thecurrent source 96 (FIG. 7) to increase its specified current by Ib. Itsoperation is similar to that of FIG. 6 with the polarities of thetransistors reversed.

The compensation circuit 100 may also be used with any type of op ampwhere the current from the Vref current source needs to be increase byIb. Such may be the case with a negative voltage regulator using an opamp with PNP transistors in its front end.

FIGS. 9A and 9B together illustrate the op amp portion of a negativevoltage regulator that adapts to a wide range of negative input voltagesVee. The Set pin 101 of FIG. 9B, providing Vref, connects to the base ofthe transistor 30 at the bottom of FIG. 9A.

In FIG. 9B, the current source 60 is the same as that shown in FIG. 5.The base current compensation circuit 104 is similar to that shown inFIG. 6 but with the current source 106 connected to the collector of thetransistor 108 rather than its emitter. The transistor 110 sets abiasing voltage for the transistor 108.

In the op amp 22 of FIG. 9A, if Vee becomes less than (Vref+Vbe), thecurrent source 33 will not operate properly. Therefore, to increase theoperating range of the regulator, another op amp 112 at the top of FIG.9A takes over when Vee becomes less than (Vref+Vbe). The op amp 112 ofFIG. 9A uses PNP transistors 114 and 116 in its differential amplifier,so the Vref only has to be more negative than ground for the currentsource 120 to operate. The base current compensation circuit 122 removesIb from the node connecting Vref to the base of transistor 116 tocompensate for the sourcing of Ib by the transistor 116.

The outputs of the op amps 22 and 112 may be applied to a buffer 124that controls a pass transistor (connected between Vee and Vout) forregulating Vout.

The portion of the regulator at the top of FIG. 9A may be conventionalsince Vref will be close to Vee. The circuit portion at the bottom ofFIG. 9A and in FIG. 9B is primarily useful for extending the range ofVout when Vout approaches ground, as described with respect to FIG. 4.

Those skilled in the art will understand the operation of the circuitsof FIGS. 9A and 9B since the circuits are similar to those previouslydescribed.

FIG. 10 illustrates the base current compensation circuit 100 applied tothe current source 96 for setting the Vref for an op amp 130 (an erroramplifier) in a switching voltage regulator 132. A pulse widthmodulation circuit 134 compares the output of the op amp 130 with asawtooth waveform and, when the signals cross, the PWM circuit 134 turnsoff the power switch in the circuit block 136. The power switch isturned on at the beginning of the next clock cycle. The power switch istypically a bipolar transistor or a MOSFET. The output voltage Vout isfed back into the op amp 130, and the duty cycle of the power switch iscontrolled to cause Vout to equal Vref. Switching power supplies arewell known and further detail is not necessary.

Many other applications of the base current compensation scheme of thepresent invention are envisioned.

The compensation circuit is not limited for use in voltage regulators.For example, the compensation may be used in any circuit requiring aprecision reference voltage despite any error currents added to orsubtracted from the current source output current. In one embodiment,the compensation circuit may be used as a reference voltage generator ina temperature sensor.

The current source need not be connected to a resistance to generate areference voltage and may be connected to another component. Forexample, the current source may be connected to a capacitor to generatea ramping signal for application to a comparator or other amplifier. Theamplifier error current is then compensated by the present invention toimprove the precision of the ramping voltage.

The error current need not be created by a bipolar transistor and may befrom any circuit that adds to or subtracts a current from the currentsource.

In all cases, the operation of the inventive compensation circuit is notaffected by the magnitude of the voltage generated at the outputterminal of the current source. Accordingly, the voltage at the outputterminal may extend to, for example, the low rail voltage, such asground.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications thatare within the true spirit and scope of this invention.

What is claimed is:
 1. A circuit comprising: a first current sourcegenerating a first current and having an output terminal, a firstvoltage being generated at the output terminal; a set resistance coupledto the output terminal of the first current source, wherein a voltagedrop across the set resistance generates a reference voltage for settinga target output level of a regulator; the output terminal also beingconnected to a control terminal of a transistor that introduces a firsterror current, wherein the first error current offsets the first voltagefrom a target first voltage; and a correction circuit connected to asecond terminal of the first current source, the correction circuitgenerating a second current approximately equal to the first errorcurrent, which modifies the first current to compensate for the firsterror current so as to cause the first voltage to be approximately thetarget first voltage, wherein operation of the correction circuit is notdependent on a magnitude of the first voltage.
 2. The circuit of claim 1wherein the circuit is a voltage regulator circuit and wherein the firsterror current is an input current into an amplifier.
 3. The circuit ofclaim 2 wherein the amplifier is an error amplifier receiving at a firstinput terminal a voltage corresponding to an output voltage of theregulator and receiving at a second input terminal a reference voltage,the reference voltage corresponding to the first voltage.
 4. The circuitof claim 1 wherein the first error current is a base current into abipolar transistor, and wherein the correction circuit comprises a basecurrent compensation circuit generating the second current approximatelyequal to the base current of the bipolar transistor for adjusting thefirst current to compensate for the base current.
 5. The circuit ofclaim 1 wherein the correction circuit is not connected to the outputterminal of the first current source so as to be unaffected by the firstvoltage.
 6. The circuit of claim 1 wherein the error current is a basecurrent of a bipolar transistor, and the correction circuit is notconnected to the base of the bipolar transistor so as to be unaffectedby the first voltage.
 7. The circuit of claim 1 wherein the circuit is apositive voltage regulator.
 8. The circuit of claim 1 wherein thecircuit is a negative voltage regulator.
 9. The circuit of claim 1wherein the circuit is a linear regulator.
 10. The circuit of claim 1wherein the circuit is an integrated circuit.
 11. The circuit of claim 1wherein the error current is an input current into an error amplifier ina voltage regulator, an output of the error amplifier being connected tocontrol a pass transistor to generate a regulated voltage approximatelyequal to a voltage at the output terminal of the first current source.12. The circuit of claim 1 wherein the first current source comprises afirst transistor in a feedback loop, wherein a conduction of the firsttransistor controls a current through a resistor to generate a voltageat a first node to match a first reference voltage, and wherein thecorrection circuit provides the second current to the first nodeapproximately equal to the first error current.
 13. The circuit of claim12 wherein the correction circuit sinks the second current from thefirst node approximately equal to the first error current.
 14. Thecircuit of claim 12 wherein the correction circuit sources the secondcurrent to the first node approximately equal to the first errorcurrent.
 15. The circuit of claim 1 wherein the first error current is afirst base current for a first bipolar transistor, wherein thecorrection circuit comprises: a second current source providing acurrent through a second bipolar transistor to generate a second basecurrent approximately equal to the first base current; and a currentmirror connected to the second bipolar transistor to generate the secondcurrent coupled to the second terminal of the first current source. 16.A method performed by a circuit comprising: generating a first currentby a first current source, the first current source having an outputterminal, a first voltage being generated at the output terminal;providing a set resistance coupled to the output terminal of the firstcurrent source such that a voltage drop across the set resistancegenerates a reference voltage for setting a target output level of aregulator; introducing a first error current at the output terminal, thefirst error current being a control current for a transistor connectedto the output terminal, the first error current offsetting the firstvoltage from a target first voltage; and generating a second current, bya correction circuit connected to a second terminal of the first currentsource, approximately equal to the first error current, which modifiesthe first current to compensate for the first error current so as tocause the first voltage to be approximately the target first voltage,wherein operation of the correction circuit is not dependent on amagnitude of the first voltage.
 17. The method of claim 16 wherein thecircuit is a voltage regulator circuit and wherein the first errorcurrent is an input current into an amplifier.
 18. The method of claim17 wherein the amplifier is an error amplifier receiving at a firstinput terminal a voltage corresponding to an output voltage of theregulator and receiving at a second input terminal a reference voltage,the reference voltage corresponding to the first voltage.
 19. The methodof claim 16 wherein the output terminal of the first current source iscoupled to a resistance, and wherein a voltage drop across theresistance generates a reference voltage.
 20. The method of claim 16wherein the first error current is a base current into a bipolartransistor, and wherein the correction circuit comprises a base currentcompensation circuit generating the second current approximately equalto the base current of the bipolar transistor for adjusting the firstcurrent to compensate for the base current.
 21. The method of claim 16wherein the correction circuit is not connected to the output terminalof the first current source so as to be unaffected by the first voltage.22. The method of claim 16 wherein the error current is a base currentof a bipolar transistor, and the correction circuit is not connected tothe base of the bipolar transistor so as to be unaffected by the firstvoltage.
 23. The method of claim 16 wherein the error current is aninput current into an error amplifier in a voltage regulator, the methodfurther comprising controlling a pass transistor by an output of theerror amplifier to generate a regulated voltage approximately equal to avoltage at the output terminal of the first current source.
 24. Themethod of claim 16 wherein the first current source comprises a firsttransistor in a feedback loop, wherein a conduction of the firsttransistor controls a current through a resistor to generate a voltageat a first node to match a first reference voltage, and wherein thecorrection circuit provides the second current to the first nodeapproximately equal to the first error current.
 25. The method of claim16 wherein the first error current is a first base current for a firstbipolar transistor, wherein the correction circuit performs the methodcomprising: providing a current through a second bipolar transistor,using a second current source, to generate a second base currentapproximately equal to the first base current; and generating the secondcurrent using a current mirror coupled to the second bipolar transistorand coupled to the second terminal of the first current source.